Pathological anatomies such as tumors and lesions can be treated with an invasive procedure, such as surgery, which can be harmful and full of risks for the patient. A non-invasive method to treat a pathological anatomy (e.g., tumor, lesion, vascular malformation, nerve disorder, etc.) is external beam radiation therapy. In one type of external beam radiation therapy, an external radiation source is used to direct a sequence of x-ray beams at a tumor site from multiple angles, with the patient positioned so the tumor is at the center of rotation (isocenter) of the beam. As the angle of the radiation source changes, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to the tumor. As a result, the cumulative radiation dose at the tumor is high and the average radiation dose to healthy tissue is low.
The term “radiotherapy” refers to a procedure in which radiation is applied to a target region for therapeutic, rather than necrotic, purposes. The amount of radiation utilized in radiotherapy treatment sessions is typically about an order of magnitude smaller, as compared to the amount used in a radiosurgery session. Radiotherapy is typically characterized by a low dose per treatment (e.g., 100-200 centiGray (cGy)), short treatment times (e.g., 10 to 30 minutes per treatment) and hyperfractionation (e.g., 30 to 45 days of treatment). For convenience, the term “radiation treatment” is used herein to mean radiosurgery and/or radiotherapy unless otherwise noted.
One challenge facing the delivery of radiation to treat pathological anatomies is identifying the target region at a particular point in time because the pathological anatomies may move as a function of the patient's breathing or other natural movements. In radiation treatment, it is useful to accurately locate and track the motion of a target region due to respiratory or other patient motions during the treatment. In order to perform radiation treatment in organs near the abdomen, for example, lungs, liver, or pancreas, it is necessary to take into account the fact that these structures move during the patient's respiratory cycle. Conventional methods and systems have been developed for performing tracking of an internal target region, while measuring and/or compensating for breathing and/or other motions of the patient.
In one conventional method, instead of prescribing a dose solely to the target region, a margin around the target region is defined so that the entire volume traversed by the target region during free breathing receives the prescription dose. Another conventional method controls the amplitude of the patient's respiration, for example, by using a restraint on the chest, so that tissue movement is reduced. A treatment margin is defined, but in this case a smaller treatment volume is used to reflect the reduced amplitude of motion.
Yet other conventional methods utilize breath holding and respiratory gating to compensate for target region movement during respiration while a patient is receiving conventional radiation treatments. Breath holding requires the patient to hold their breath at the same point in each breathing cycle, during which time the tumor is treated while it is presumably stationary. A respirometer is often used to measure the tidal volume and ensure the breath is being held at the same location in the breathing cycle during each irradiation moment. This method takes a relatively long time and often requires training the patient to hold their breath in a repeatable manner.
Respiratory gating involves a process of measuring the patient's respiratory cycle during treatment and then turning the radiation beam on only for a predetermined part of the patient's breathing cycle. Respiratory gating does not directly compensate for motions that result from breathing. Rather, radiation treatment is synchronized to the patient's breathing pattern, limiting the radiation beam delivery to times when the tumor is presumably in a reference position. Respiratory gating may be quicker than the breath holding method, but also may require the patient to have many sessions of training over several days to breathe in the same manner for long periods of time. Conventional respiratory gating also may expose healthy tissue to radiation before or after the tumor passes into the predetermined position. This can add an additional margin of error of about 5-10 millimeters (mm) on top of other margins normally used during treatment. However, the prescription volume can usually be smaller than that using free breathing without gating. These conventional methods are limited by the patient's ability to perform breathing functions in a consistent manner over multiple treatment sessions.
Another conventional method of dealing with the motion of a target region during radiation treatment involves the image tracking of fiducial markers that are placed in or near the target region. The position and motion of the fiducial markers is correlated with the position and motion of the target region so that real-time correction of the position of the treatment beam to follow the motion of the target region may be realized.
Each of these techniques has its advantages and drawbacks. Without restraint or gating, a fast treatment is possible that is comfortable for the patient. However, especially in regions where respiratory motion is large, for example, near the diaphragm, this approach necessitates the irradiation of a volume of tissue substantially larger than the target region. Controlling respiratory amplitude can make treatment uncomfortable, and gating causes an increase in treatment time. Performing real-time correction according to the movement of fiducial markers implanted in the target region allows a conformal dose distribution to be delivered quickly. Nevertheless, this method does have a disadvantage that it requires invasive fiducial implantation. Real-time correction according to the movement of fiducial markers also requires a radiation delivery device that can be moved quickly and accurately. One such radiation treatment system is the CYBERKNIFE® system developed by Accuray Incorporated of California. By mounting a compact X-band linear accelerator on a robot arm assembly, the CYBERKNIFE® radiation treatment system can perform real-time compensation for respiratory motion.
One conventional treatment planning approach using a CYBERKNIFE® radiation treatment system utilizing inverse planning techniques is as follows. First, a target region and critical structures to be avoided are delineated on a CT scan, or a set of CT slices of a volume of interest (VOI) in the patient. More specifically, a three-dimensional (3D) CT scan is composed of a three-dimensional model of a volume of interest (e.g., pathological anatomy bearing portion of the body) generated from a collection of two-dimensional (2D) CT slices, with each slice representing a different position in space (for example, a different position along the inferior-superior axis of the patient). In CT scanning, numerous x-ray beams are passed through a volume of interest in a body structure at different angles. Then, sensors measure the amount of radiation absorbed by different tissues. As a patient lies on a couch, an imaging system records x-ray beams from multiple points. A computer program is used to measure the differences in x-ray absorption to form cross-sectional images, or “slices” of the head and brain. These slices are called tomograms; hence the name “computed tomography.”
Once the target region and critical structures have been delineated, dose constraints may then be applied by a medical physicist to these target regions and critical structures. The medical physicist specifies the minimum dose, and optionally the maximum dose, to the tumor and the maximum dose to other healthy tissues independently. The treatment planning software then selects a set of treatment beam parameters (e.g., direction, total number of beams and energy of the beams) in order to achieve the specified dose constraints. Next, the dose constraints may be altered, tuning structures may be added, and the treatment plan re-optimized until the dose distribution is acceptable. The finalized treatment plan is then sent to a treatment delivery system.
One article entitled, 4-Dimensional Computed Tomography Imaging and Treatment Planning, Paul Keall, Seminars in Radiation Oncology, Vol 14, No 1 (January), 2004; pp 81-90, discusses the idea of radiotherapy planning using a 4D CT image. The planning optimization step is described in this article as a set of separate 3D optimization steps performed on each of the 3D CT images making up the 4D CT.